The present invention relates to clamping fixtures, such as clamping fixtures put to use for clamping workpiece pallets provided with a tool or workpiece positionally defined, the chuck usually being fixedly located on a work table of a machine tool or a press whilst the workpiece pallet is repeatedly releasably located precisely positioned at the chuck.
Whilst with known clamping fixtures the resulting compression forces are transmitted mostly via large contact surfaces provided for this purpose, tensile forces in general have to be handled by a clamping mechanism provided with clamping members. Such a clamping mechanism usually features a plurality of spring-loaded clamping balls which when the workpiece pallet is clamped in place engage a clamping surface of a clamping post secured to the workpiece pallet or directly engage the workpiece pallet itself. Although clamping fixtures of this kind have a proven record of success, for some applications it would be an advantage when the workpiece pallet is able to handle higher tensile forces. One example of such an application is a powder press where a ram for compacting the powder is secured to the workpiece pallet. Where not only the face of the ram but especially also its inner and/or outer shell surfaces is/are involved in the actual molding action, correspondingly high withdrawal forces need to be applied to retract the ram from the die so that the ram can be withdrawn axially from the highly compacted mass of powder.
Thus, it may certainly be the case that for withdrawing the ram more than 30% of the compacting force needs to be applied in the tensile direction. In other words, when, for example, to produce a compact of ferrous or ceramic powder a compacting force of 100 tonnes needs to be applied, to remove the ram 30 tonnes or more need to be applied in the tensile direction. Where the ram is secured to a workpiece pallet, these forces must, of course, also be transmitted from the workpiece pallet to the chuck. It will be appreciated that such high withdrawal forces cannot be handled by a conventional ball clamping mechanism because due to the very small contact surface of the balls the surface compression would be much too high.
Known from EP-A-0 255 042 is a clamping device for a tool of a machine tool comprising a chuck and a tool holder. The chuck comprises two pairs of centering rails protruding from its underside which in the X and Y direction are provided with contact surfaces for aligning the tool holder Provided furthermore are four posts protruding from the surface of the chuck which are responsible for aligning the tool holder in the Z direction. The tool holder comprises a flat surface defined to contact the faces of the aforementioned posts. Provided furthermore in the tool holder are two pairs of grooves aligned with the centering rails and featuring flexible lips provided to contact the rails. In conclusion, the tool holder features a center bore for receiving a draw bar with the aid of which the clamping force needed to center the workpiece pallet correctly positioned is transmitted. In this arrangement the chuck comprises a centrally located ball lock which cooperates with said draw bar. It will be appreciated that only relatively small tensile forces can be transmitted via the draw bar and the ball lock.
Described in EP-A-0 827 806 is a clamping cylinder for clamping a pull-in nipple. The clamping cylinder is provided with a central bore serving to receive the pull-in nipple. The clamping cylinder features a first outer piston and a second piston located radially within the first. Disposed between the two pistons is a plurality of clamping members, termed locking pistons serving to clamp the pull-in nipple in place. These locking pistons engage a lug arranged at the front end of the pull-in nipple, several locking pistons being arranged evenly distributed circumferentially. The two pistons are axially movable together with the locking pistons which are additionally adjustable radially. The outer piston is provided with an inclined surface and loaded axially by means of compression springs such that it is caused to urge the locking pistons radially inwards by the inclined surface. The locking pistons are configured as plunger members. Piston release is done hydraulically. One serious drawback of this clamping cylinder is that a pallet pull-in nipple is pliable in the extension direction and can only handle relatively small forces since the locking pistons come into contact with only a comparatively small contact surface of the pull-in nipple and the piston locating the pull-in nipple via the locking pistons in the Z direction is loaded only by the compression springs in the extension direction.
Known furthermore from EP-A-1 068 919 is a clamping apparatus capable of handling comparatively high tilting and torque loads. The clamping apparatus comprises a chuck to be located in the working range of the machine tool and a workpiece carrier means for mounting on the chuck for clamping thereto. First positioning means on the chuck and second positioning means on the workpiece carrier means cooperate paired as alignment members for positioning the workpiece carrier means in the X, Y and Z direction. To clamp the workpiece carrier means in the position defined by the positioning means on the chuck, a clamping device is likewise provided. The latter comprises sited along a circular ring a plurality of clamping balls the active lines of which in their clamping force are located substantially on or in the region of an imaginary cylindrical shell parallel to the Z axes cutting the first and second positioning means. Compressive loading is transmitted from the workpiece carrier means to the chuck primarily via Z contact surfaces at which the workpiece carrier means comes to rest when clamping the chuck. Although, on the one hand, such a clamping apparatus is capable, of handling comparatively high compressive loads and also of transmitting high torques as well as tilting moments, on the other, transmission of tensile forces is limited for reasons as recited above the outset.